The present invention relates to thermal protection of disc brake components.
The application of a brake pad against a rotating brake disc results in the generation of very high temperatures. Although a large portion of the heat generated by the friction between the brake pad and the brake disc is dissipated by heat transfer directly from the brake disc to the air, a significant amount of heat passes from the brake pad into the structure of the brake caliper. In particular, undesirably high amounts of heat may be transferred to heat-sensitive components within the brake caliper.
As shown in FIG. 1, a typical disc brake 100 uses at least one moveable piston or tappet 110 to exert a braking force on a brake disc 120 via a brake pad 130. The disc brake 100 may include two brake pads 130 opposed to each other on either side of the brake disc 120 (not shown). The brake caliper 140 straddles the brake disc 120, with the brake pad 130 located between the tappet 110 and the brake disc 120. When the brake pad 130 is in a new condition, the tappet 110 is in a fully retracted position at a maximum distance from the brake disc 120, as shown in FIG. 1. In order to push the brake pad 130 against the brake disc 120, the tappet 110 moves toward the brake disc 120. As the brake pad 130 is used, the friction between the brake pad 130 and the brake disc 120 causes the friction material of the brake pad 130 and the brake disc 120 to wear down. As the thickness of the brake pad 130 decreases, the tappet 110 moves closer to the brake disc 120 to maintain a desired maximum clearance between the brake pad 130 and the brake disc 120 in order to minimize brake free-play. Eventually the tappet 110 reaches a fully extended position when the brake pad 130 and the brake disc 120 are in a maximum wear condition, as shown in FIG. 2.
In order to ensure the performance of the disc brake 100 over its service life, the internal caliper components must be protected from environmental influences, such as dust, chemicals, gases, and water. Typically a bellows 150 is provided to seal the annular gap between the brake caliper 140 and the tappet 110, and to prevent dust, chemicals, gases, and water from entering the internal mechanism of the brake. The bellows 150 has a first surface 151 that is affixed to the caliper 140, and a second surface 152 that is affixed to the tappet 110. The first surface 151 of the bellows 150 may be directly connected to the caliper 140 or to a component outside of the caliper 140, such as a cover plate bolted to the caliber 140 (not shown). The bellows 150 extends and retracts with the tappet 110 as the tappet 110 advances toward the brake disc 120 and returns to its rest position. In this way the bellows 150 maintains the seal between the brake caliper 140 and the tappet 110 during all operational conditions.
The bellows 150, which is typically made of silicon rubber, is particularly sensitive to high temperatures. Contact between the bellows 150 and the portion of the tappet 110 immediately adjacent to the brake pad 130 (i.e. the portion of the tappet 110 reaching the highest temperatures during brake application) must be avoided, because this portion of the tappet 110 may reach temperatures that can melt or damage the rubber bellows 150. Such contact is particularly likely when the tappet 110 is in or near the fully retracted position as shown in FIG. 1, when the bulk of the retracted the bellows 150 is gathered near the tappet 110. Further, the folds of the bellows 150 do not collapse uniformly during retraction. Instead, the folds collapse stepwise, and single folds can resist the collapsing movement by remaining in an inclined position for an extended period of time. This increases the risk of contact between local areas of the bellows 150 and the tappet 110 as the tappet 110 moves away from the brake disc 120.
The bellows 150 is also vulnerable to convective heat transfer from the braking process. For example, when the tappet 110 is in the fully extended position as shown in FIG. 2, the bellows 150 is completely unfolded, such that its full wall length is exposed to environmental attack and heat from the braking process. Further, the bellows 150 typically sees higher temperatures when the tappet 100 is fully extended (i.e. when the brake pad 130 and the brake disc 120 are in the maximum wear condition), as compared to when the brake pad 130 and the brake disc 120 are new. This is due to the reduced heat capacity of the brake disc 120 and the brake pad 130 caused by their reduced thicknesses as well as the reduced thermal insulation effect of the thinner, worn out brake pad 130. Therefore, the bellows 150 is particularly vulnerable to thermal degradation when the tappet 110 is in the fully extended position.
Previous disc brake designs have included mechanisms for protecting the bellows from heat generated during the braking process. For example, the rubber bellows may be enclosed with a metallic spiral spring enclosure. However, because the metal has a high heat conductivity, the metal may melt or damage the rubber bellows upon contact. Further, it is difficult to prevent contact between the metallic enclosure and the bellows, due to the small packaging space and the uncontrollable deformation of the bellows and the metallic spring. In addition, the metallic enclosure has a poor strength of shape, especially along the lateral direction and in the fully expanded position. This can cause individual coils to skip or jam, which adversely affects the protection function of the metallic enclosure. It can also compromise the full release of the brake and cause a running clearance reduction with the risk of a hot running brake. Further, in this state it is not possible to completely retract the tappet from the brake disc to change the brake pad, due to the increased block height of the spiral spring.
U.S. Pat. No. 7,267,207 discloses a disc brake in which the rubber bellows are replaced with metallic bellows. However, this requires a high cost for caliper design changes, because metallic bellows are made of very thin-walled and high-grade stainless steel, and the metallic bellows are costly. In addition, relatively high forces are needed to deform the metallic bellows during extension and retraction of the tappet.
U.S. Publication No. 2001/0047913 discloses a disc brake that includes a pressure plate and a heat shield on the back plate of the brake pad. However, this provides limited protection against convective heat transfer, because the sides of the bellows are still exposed. Also, additional space is needed for the pressure plate and heat shield, and there is a significant additional cost to provide the pressure plate and heat shield, and their connection to the tappet.
U.S. Publication No. 2006/0175155 discloses a disc brake that includes an insulation disc on the tappet. The insulation disc is only effective against conductive heat transfer, and does not prevent contact between the bellows and the tappet. Also, depending on its elasticity, the insulation disc may increase the stroke demand of the brake.
In addition, other disc brake designs have included a heat shield that partially covers the bellows. For example, U.S. Pat. No. 4,431,090 discloses a rubber ring that deforms when impacted by the piston, U.S. Pat. No. 3,592,303 discloses an elastic heat shield that expands circumferentially as the plunger extends, and European Patent No. EP 1 972 821 B1 discloses a metallic heat shield that is inserted into an end of the bellows. However, none of these disc brake designs provides a heat shield that protects the entire exposed surface of the bellows throughout the operation of the disc brake.
Accordingly, there is a need for an improved disc brake in which the bellows 150 is protected against excessive heat generated during the braking process. In particular, the apparatus should advantageously protect the bellows 150 from heat throughout the entire operating range of the tappet 110.